C. elegans slows its locomotion rate in response to food using either of two distinct slowing mechanisms. When well-fed, C. elegans senses a bacterial lawn (the food source) using mechanosensory cues and requires dopamine signaling for slowing (the basal slowing response). When deprived of food for 30 minutes or more, C. elegans slows to a greater extent upon entry into a bacterial lawn and requires serotonin signaling (the enhanced slowing response). The molecular determinants of the well-fed and food-deprived states and the mechanisms by which animals switch between these states are unknown. Mutagenesis and RNA interference screens for well-fed animals that fail to exhibit normal basal slowing will identify molecules needed for sensory transduction and modulation of dopaminergic signaling. Mutagenesis and RNA interference screens for animals that fail to suppress basal slowing when food-deprived will identify molecules needed to switch between basal and enhanced slowing states. Genetic, molecular, and physiological characterization of molecules identified in these screens will provide insights into dopaminergic modulation of neuronal circuits in an intact nervous system.